Someone did something like that in an a studio at art school once, the note read "please leave thanks". It stayed up for weeks but one day someone scattered a lot of little pieces of paper with the word "thanks" written on it around the note.

^^ guessed well..but the strange thing was that nobody took much notice of the extra contribution, even though it turned out that the original main notice was intended as an art piece..so I guessed either the people at my art school were transcendentially cool, or that they were collectively so dense as to be unable to get it. Its hard to tell with a crowd like that.